This invention relates in general to tensioning of risers used in oil production and exploratory drilling and, more specifically, to an improved riser tensioner assembly for such use.
Off shore oil drilling and production operations are conducted through a pipe, called a riser, running from the subsea wellhead to the surface platform or floating vessel In order to support the weight of these risers and to control the stresses induced by ocean currents and vessel motions, the upper end of the riser is connected to a tensioning device. This riser tensioner maintains a predetermined range of tension throughout the range of vertical and lateral motions of the drilling or production rig.
The conventional approach to tensioning risers is to use a combination of a hydraulic or pneumatic mechanical cylinder, pressurized using a compressed gas, to apply the tensioning forces to the riser. Each riser tensioner is located on a deck of the floating platform or floating vessel and structurally connected through its cylinders to the riser. The cylinders may be connected to the risers with wire rope or chain or directly connected through cylinder rods. The pressurized gas volume is typically contained in a separate pressure vessel referred to as an "accumulator", positioned alongside the cylinder, which supplies sufficient gas volume to act as a gas spring. This combination of cylinder and accumulator acts to compress or expand the gas in response to vessel or riser movements, maintaining a relatively uniform tension level in the riser.
This traditional approach to riser tensioning using separate cylinders and accumulators acting together has worked satisfactorily in most cases, but is not space efficient. The deck area at the platform or vessel required for each riser and its tensioner is significant and maximum utilization of the space on the vessel or platform is economically significant and can limit the number of wells that can be accommodated without enlarging the platform.
In addition, safety considerations related to personnel, investment, and ecological considerations have driven regulatory agencies to require protection of critical equipment from destruction by fire. Risers and tensioners have typically been designated as critical components that must continue to remain safe during an accidental fire.
Protection of a riser tensioner from fire in the past was generally accomplished by a water deluge system; under new requirements the tensioner is protected using heat and flame insulation materials. For a given fire hazard scenario, the selection of these materials is a function of the exposed surface area. In the typical riser tensioner arrangement with separate cylinder and accumulator there is a high exposed surface area requiring a substantial amount of insulation. This adds significant cost and weight to each tensioner unit.
The insulation is used to delay the transmission of heat to the metal parts of the tensioner, keeping them from heating up and softening. The rate of increase in metal temperature is also determined by its size (i.e., mass). The smaller the mass of a tensioner, the faster the temperature will rise as a given amount of heat passes through the protective insulation. Thus, the lower the ratio of insulation area divided by the protected mass, the longer it will take for the metal temperature to rise, and the better the system will perform during a fire. For a typical fire protected cylinder, the highest ratio of jacket area to protected metal is the extended rod of the cylinder; thus, it is the most critical component and is most likely to heat up to a softening point and eventually fail.
A number of different riser tensioner systems have been developed. Typical of such systems are those disclosed by Widiner et al in U.S. Pat. No. 4,379,657 and by Stevenson in U.S. Pat. No. 4,215,950. Widner et al disclose a tensioner having a tensioner with separate accumulator and cylinder, having the weight and volume problems discussed above. Stevenson mounts a tensioner on a gimbal arrangement that may have some alignment advantages but which increases the weight and volume problems.
Jaqua, in U.S. Pat. No. 4,799,827 discloses a riser tensioner in which the accumulator surrounds the cylinder. While this arrangement reduces the volume of the tensioner, a solid piston rod is used that will accumulate heat in a fire situation, as discussed above, and fully effective heat insulation is not provided.
A complex hydropneumatic cable tensioner is described by Jordan in U.S. Pat. Nos. 4,638,978 and 4,540,159. High pressure gas is provided to an oil filled accumulator which in turn acts on the cylinder piston This design seems to be primarily concerned with avoiding tensioner damage in the event of sudden release of cable forces on the tensioner. With complexity comes an increased likelihood of failures. An effective heat insulation system for fire protection is not provided.
Thus, there is a continuing need for improved riser tensioners for use with off shore oil drilling and production facilities, which have lower volume and mass, improved simplicity and reliability, lower lateral areas and improved resistance to damage from fire.